Hi Peter. I am working on a flight planner, that you might find useful. The plugin calculates ground speed, from your IAS and Wind speed at your chosen alt, and determines the corrected ETE.
Can I ask: What is your airspeed ? (74kph). What are your waypoints?
I use the sfc temp +3 degree for now as an approx - probably too much in the morning or when the cloud cover is important.
The surface temperature reported by the model does not seem to work better (I may want to give it an another try later).
Does it answer your question or is the question more about the temperature gradient ?